refactor!: Pass UpdateConnectedExternalGroup request body by value via new UpdateConnectedExternalGroupRequest

[JamBalaya56562]

Continues the request-body-by-value work in #3644, this time for TeamsService.UpdateConnectedExternalGroup.

The method reused the ExternalGroup response type as its request body, but per the docs the endpoint accepts exactly one body parameter, group_id, and it's required. The other four fields (group_name, updated_at, teams, members) are response-only and are ignored by the server, so the signature didn't tell callers what to fill in. The existing test shows this: it builds the request with only GroupID set, while the response expectation populates all five fields.

So this adds a dedicated request type, following the same approach as PullRequestSubmitReviewRequest in #4406:

type UpdateConnectedExternalGroupRequest struct {
	GroupID int64 `json:"group_id"`
}

GroupID is a non-pointer int64 since it's required, the body is passed by value, and the type is removed from the body-allowed-pointer-types allowlist.

Notes:

• ExternalGroup is unchanged — it stays the response type for GetExternalGroup, ListExternalGroups, ExternalGroupList and the iterator, so its fields keep their pointer semantics.
• The method verb Update already matches the docs operation name, so no rename.

Verified with go build ./..., go vet -tags integration ./test/integration/, gofmt, the full ./github/ test suite (UpdateConnectedExternalGroup and the generated GetGroupID at 100%), and custom-gcl (no paramcheck findings after removing the allowlist entry).

Updates #3644

BREAKING CHANGE: TeamsService.UpdateConnectedExternalGroup now takes a new UpdateConnectedExternalGroupRequest (with non-pointer GroupID) by value instead of *ExternalGroup.

cc @jvm986 — flagging for #3644 coordination; this is in the teams service, so it shouldn't overlap with the Issues work.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.54%. Comparing base (4d45b7c) to head (e00ee2b).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #4425   +/-   ##
=======================================
  Coverage   97.54%   97.54%           
=======================================
  Files         194      194           
  Lines       19780    19780           
=======================================
  Hits        19294    19294           
  Misses        268      268           
  Partials      218      218           

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[gmlewis]

Thank you, @JamBalaya56562!
LGTM
Awaiting second LGTM+Approval from any other contributor to this repo before merging.

cc: @stevehipwell - @alexandear - @Not-Dhananjay-Mishra